1. Field of the Invention
The present invention relates to lumen output control of a light source. More particularly, the invention provides a method and system for increasing and decreasing a ballast output power, which is connected a light source, to provide a constant light output during the life of the light source.
2. Description of the Related Art
Over time, the lumen output of a lamp continually decreases Lumen output can be defined as a unit of luminous flux equal to the light emitted in a unit solid angle by a uniform point source of one candle intensity. As related to power, a lumen is 1/683 watts of radiant power at a frequency of 540×1012 Hertz. The lumen output degradation in the lamp can occur for a variety of reasons, for example, lamp lumen depreciation, the lamp's interaction with a ballast, supply voltage variations, dirt or dust on the lamp, and the ambient temperature in a fixture. FIG. 1 illustrates a lumen degradation curve for a typical quartz metal halide high intensity discharge (HID) lamp that uses a conventional ballast. FIG. 1 is a chart 100 illustrating two curves in relation to an X-axis 102 (lamp operating hours) and a Y-axis 104 (lumens per lamp watt). The curve 106 illustrates the degradation curve for a magnetic constant wattage autotransformer (CWA) lamp and the curve 108 illustrates a degradation curve for a Prismatron™ lamp. As lamp operating hours increase for the lamp, the lumen output of the lamp decreases.
The decrease in lumen output occurs due to a variety of processes that occur within the lamp. One factor contributing to this decrease is a loss of chemicals that contributing to light output. These chemicals can be lost through portions of the lamp structure, for example, an arc container Another factor contributing to light degradation is metal being deposited on an arc tube wall of the lamp. An HID lamp is started by applying a very high voltage across an arc tube to break down high pressure gasses within the lamp into a conduction state. Following this breakdown, high current normally flows across a relatively low-voltage arc that heats the electrodes, which subsequently enter into thermionic emission. This tends to eject molecules of the metal electrode material that eventually condense on the wall of the arc tube, causing “blackening” and lowering the light transmission of the arc tube.
Due to such degradation in lumen output, many lighting applications are designed using a mean light level. The mean light level, or lamp's lumen, is defined when a HID lamp is at forty percent of its rated life. Typically to achieve a minimum light level emission, a lighting system designer will design a lighting system at the mean light level. Once the lamp is at a point past the mean light level, replacement of the lamp is usually necessary to maintain a desired light output level.
In HID applications, a ballast is used to control the operating power delivered to a lamp. FIG. 2 is a block diagram 200 illustrating a typical ballast 202. The ballast 202 regulates the power to the lamp 204 which is received as an input voltage from a power source (not shown). The ballast 202 also provides proper starting conditions for the lamp 204 at start-up
Some ballast designs use magnetic transformers. As a result, the output level of a lamp cannot be varied and is limited to an output of full power or some fixed output level lower than full power. Other ballast designs, such as electronic ballasts, provide for continuous variation of lamp voltage between full power and a predetermined lower limit.
However, a problem with conventional ballast systems, using the mean light level to set a desired lamp output, is that the ballast initially consumes additional power for the time period prior to achieving the mean light level. Powering the lamp at full output prior to achieving the mean light level causes an output higher than is necessary which consumes more power than necessary to provide the desired light output.
Accordingly, there is a need and desire for a ballast having a power regulation technique for outputting power to a lamp, which will create a constant lumen output from the lamp, thereby decreasing the power consumption of the lamp system.